Without seeing a picture of it, its hard to put an exact value on it.
Depending on condition, finish, box/papers, and how many mags, I'd say its worth anywhere between $350 to $450.
I have three P239s - a P239 SAS 9mm, P239 9mm DA/SA, and a P239 TT .40S&W/.357sig DA/SA.
I only carry the SAS because of the DAK (double action only) trigger. It is my main summer time CCW because of how slim the single stack mag makes the gun. (It's sitting with me in my Sig Tac coat as I post this.)
They are all excellent shooters. Some people find them to be heavy and lacking in capacity (8 + 1 - 9mm, 7 + 1 - .40/.357sig), but for me it's the smallest gun that I can shoot really well.
That being said, out of all of my Sigs, they are the ones that I like the least because I have big hands.
As far a reliability goes, its a Sig and its built like a tank. If you take care of it, it'll last you a lifetime.
Since it is a friend selling it, take it to the range and make sure its a good fit for you before you buy it.
Good luck!